Volume 111, Issue 4, Pages 732-742 (August 2016) The Conformation of Interfacially Adsorbed Ranaspumin-2 Is an Arrested State on the Unfolding Pathway  Ryan J. Morris, Giovanni B. Brandani, Vibhuti Desai, Brian O. Smith, Marieke Schor, Cait E. MacPhee  Biophysical Journal  Volume 111, Issue 4, Pages 732-742 (August 2016) DOI: 10.1016/j.bpj.2016.06.006 Copyright © 2016 Terms and Conditions

Figure 1 (A) Cartoon of the wild-type Rsn-2 atomistic structure, highlighting the hydrophobic side chains on the N-terminal tail (residue IDs 1–15, in blue), and the α-helix (purple) and β-sheet (yellow) structures. The two extra disulphide bonds in the 2C-C mutant are represented by the red dotted segments. (B) The same information as (A), but now coarse-grained (see Materials and Methods for the description of the model). We only represent the backbone beads and the side-chain beads on the N-terminal tail; all other side-chain beads are omitted for simplicity. (C) The α-helix and β-sheet and their hydrophobic side chains, highlighting the buried hydrophobic core between these two structural elements. To see this figure in color, go online. Biophysical Journal 2016 111, 732-742DOI: (10.1016/j.bpj.2016.06.006) Copyright © 2016 Terms and Conditions

Figure 2 Kinetics of Rsn-2 adsorption to the air-water interface. (A) Interfacial tension, Γ, was measured using pendant drop tensiometry. Figure shows Γ as function of Rsn-2 concentration: 0.0012 mg mL−1 (black), 0.005 mg mL−1 (red), 0.01 mg mL−1 (blue), and 0.05 mg mL−1 (pink). (B) Black data points correspond to the theoretical time that Rsn-2 would take to adsorb to an interface with a surface concentration of 0.95 mg m2. This surface concentration corresponds to a ΔΠ value of 2 mN m−1. The red data points are the times extracted from (A) for a change in surface pressure for the bare interface. The results show that initial adsorption of Rsn-2 is well described by a diffusive model. To see this figure in color, go online. Biophysical Journal 2016 111, 732-742DOI: (10.1016/j.bpj.2016.06.006) Copyright © 2016 Terms and Conditions

Figure 3 Estimate of the size of Rsn-2 at an interface. Three independent area-pressure isotherms were obtained using a Langmuir trough. The turnover in the isotherm corresponds to surface saturation and was determined by finding the second derivative of the isotherm (39). To see this figure in color, go online. Biophysical Journal 2016 111, 732-742DOI: (10.1016/j.bpj.2016.06.006) Copyright © 2016 Terms and Conditions

Figure 4 Behavior of Rsn-2 variants at the air-water interface. (A) Pendant drop tensiometry of wild-type Rsn-2 (black), N-terminal deletion (red), and 2C-C mutant (blue). All concentrations were 0.01 mg mL−1. (B) Surface pressure/area isotherms for wild-type Rsn-2 (black) and the 2C-C mutant (red). This result demonstrates that the 2C-C mutant occupies a significantly smaller area at the interface compared to wild-type. To see this figure in color, go online. Biophysical Journal 2016 111, 732-742DOI: (10.1016/j.bpj.2016.06.006) Copyright © 2016 Terms and Conditions

Figure 5 Structure of Rsn-2 at the interface. The spectrum of wild-type Rsn-2 in solution (black), RIME of wild-type Rsn-2 (red), and RIME of 2C-C mutant (blue). The spectra are normalized assuming all protein is present in the emulsion. To see this figure in color, go online. Biophysical Journal 2016 111, 732-742DOI: (10.1016/j.bpj.2016.06.006) Copyright © 2016 Terms and Conditions

Figure 6 Energetics of adsorption of Rsn-2 to an ideal interface. (A) Schematics of the coarse-graining (CG) from the all-atom NMR structure (AA), and typical Rsn-2 configuration at different adsorption stages: in bulk (state 0, corresponding to the native structure); with the flexible N-terminal region adsorbed at the interface (state 1); and fully adsorbed and partially unfolded in side-view (top) and top-view (bottom) (state 2). The N-terminal tail (residue IDs 1–15) is shown in blue, the α-helix (16–38) in purple, the β-sheet (45–88) in yellow, and the loop (39–44) and the C-terminal tail (89–96) are in gray. The interface is represented by the black wavy line. For the coarse-grained structures we only show the Cα beads and not the side chains. (B) Energy of interaction with the interface for a typical adsorption event, showing the energetic contributions from the N-terminus (residues 1–15; dashed cyan line) and the globular region of the protein (residues 16–96; dashed blue line) to Rsn-2 adsorption (purple line). (C) Fraction of native contacts corresponding to the α-helix and the β-sheet (green line) and those involved in the formation of the hydrophobic core (dashed pink line). The first adsorption step involves no loss of native contacts, whereas the second step involves the disruption of the hydrophobic core while the secondary structure elements are conserved. To see this figure in color, go online. Biophysical Journal 2016 111, 732-742DOI: (10.1016/j.bpj.2016.06.006) Copyright © 2016 Terms and Conditions

Figure 7 Energy of interaction with the interface (A) and fraction (B) of hydrophobic core native contacts for typical adsorption events observed for the d1–15 (yellow dotted lines) and 2C-C mutant (brown dashed lines). The deletion of the flexible tail reduces the interaction energy of the first adsorption state. The addition of the two disulphide bridges does not affect the first step but abolishes the unfolding of the hydrophobic core resulting in a significant reduction of the interaction energy. To see this figure in color, go online. Biophysical Journal 2016 111, 732-742DOI: (10.1016/j.bpj.2016.06.006) Copyright © 2016 Terms and Conditions

Figure 8 Free energy of the first step of adsorption for wt (green), d1–15 (yellow), and 2C-C (brown) Rsn-2 as a function of the distance between the interface and the center of mass of the section of the protein common to all mutants (residues 16–96). The disulphide bridges have no effect on the profile, whereas the tail deletion reduces the free energy of adsorption to approximately one-half. To see this figure in color, go online. Biophysical Journal 2016 111, 732-742DOI: (10.1016/j.bpj.2016.06.006) Copyright © 2016 Terms and Conditions

Figure 9 (A) Free-energy landscape of the second step of adsorption for wt (left), d1–15 (center), and 2C-C (right) Rsn-2 as a function of the distance between the interface and the center of mass of Rsn-216–96 and the native contact fraction. (B) Comparison between the free-energy landscape of wt Rsn-2 at the interface at ambient temperature (T = 0.9 Tf, left) and in bulk at the folding temperature (T = Tf, right), as a function of secondary structure and hydrophobic core contact fractions. (Black, green, and red curves) Three representative unbiased pathways between the two free-energy minima (corresponding to folded and unfolded states). The inset in (B) shows the one-dimensional free-energy profile of the second adsorption step as a function of the fraction of native core contacts, highlighting the barrier of ∼6 kBT that has to be overcome for the unhinging transition. To see this figure in color, go online. Biophysical Journal 2016 111, 732-742DOI: (10.1016/j.bpj.2016.06.006) Copyright © 2016 Terms and Conditions